In order to comply with ever restrictive emission regulations for limiting pollutants, real-time diagnostic techniques for combustion process monitoring in internal combustion (IC) engines are widely used. Information about the quality of combustion may provide important data for diagnosing the working of the engine and may be profitably used for advanced electronic engine controls. These controls are aimed to ensure a good combustion quality in any condition of engine operating by means of real time removal of combustion anomalies, thus improving performances and reducing toxic exhaust gas emissions. Two important combustion anomalies in IC engines are misfire and partial burning. Misfire and partial burning are terms used to indicate an absent and a weak combustion in a combustion cycle. In IC engines these phenomena generally occur when the incoming air/fuel mixture is excessively diluted (with air or with exhaust gas recycled) or when faults occur in the ignition system.
When, in a cylinder of an IC engine, combustion does not occur or occurs only incompletely, unburned fuel enters the exhaust system and eventually burns in the hot catalytic converter.
The released heat may damage or destroy the catalytic converter by thermal overloading. Moreover, misfire and partial burning events lead to instability of the engine and to a rapid increase of hydrocarbon emissions. Current emission regulations force engine manufacturers to equip cars with systems able to detect engine misfires and to alert the driver whenever the misfire rate has the potential to affect the engine after-treatment system. For all these reasons, detection of misfire is a critical issue for electronic engine control systems and several methods have been proposed and used to this aim.
A method largely used to evaluate misfires for on-board diagnosis purposes is based on the analysis of the rotational velocity of the engine by means of signal coming from a crankshaft inductive speed sensor. A misfire event is correlated to characteristic variations in the crankshaft speed: these speed fluctuations are used as misfire indicators for misfire diagnosis. However, the crankshaft speed fluctuation method has difficulties in detecting misfires in some particular conditions.
For example, rough roads can make the engine velocity profile appear as if misfire events were occurring. Moreover, the rotational speed, measured on the crankshaft, is influenced by combustion in all cylinders, thus performances of these misfire detection methods are relatively poor when the total mass of inertia, engine speed, or the number of cylinders increase, because the relative acceleration difference between normal combustion and misfire becomes almost imperceptible under these circumstances.
All these considerations lead to sophisticated hardware and software for analysis and filtering speed data. Alternative methods based on ionization analysis are considered an efficient approach to misfire detection. Their main advantage is exploitation of a component already present inside the combustion chamber, namely the spark plug, for ion-current sensing.
However, the most sensitive and reliable method for misfire and partial burning detection involves direct pressure measurement inside each cylinder of the engine, because the parameter widely considered as the most important parameter for the evaluation of combustion quality is the pressure in the cylinder.
It has been observed that, in case of misfire events, the in-cylinder pressure cycle presents a typical waveform as depicted in FIG. 1. When a misfire event occurs, the in-cylinder pressure peak is located at 0 crank angles (C.A.), that is at the Top Dead Center position of the piston, for every engine working condition. This means that the location of the pressure peak (LPP) is always equal to 0 C.A.
In case of partial burning, due for example to a highly diluted air-fuel mixture, the duration of the combustion process increases and there is no time to complete the combustion before the exhaust gas valve opens. As a consequence, the combustion pressure cycle has a typical shape as depicted in FIG. 2, wherein the pressure peak is significantly shifted to the right of the top Dead Center position (0 C.A.), much more than in the case of “normal combustion”.
The above described characteristics of the in-cylinder pressure cycle, during combustion anomalies, demonstrate that the LPP is an important parameter for real time diagnosing the occurrence of misfires or of partial burning events. In particular:                LPP is equal to a zero crank angle in case of misfire events;        LPP falls in a certain angular range of the crank position in case of normal combustion; and        LPP is greater than a certain angular value in case of partial burning.        
A drawback of this technique is that it is difficult and relatively expensive to install pressure sensors in the combustion chamber. Moreover, the pressure sensor installed in the cylinder must be capable of withstanding high temperatures and pressures without being damaged. For all these reasons, diagnostic techniques based on cylinder pressure analysis are currently limited to research applications.
Compared to the use of combustion pressure sensors, non-intrusive diagnostic techniques offer several advantages because the sensors are generally placed out of the combustion chamber and thus no structural modifications of the engine are required. Moreover, these sensors do not need to withstand very high pressures and temperatures, therefore they may be of relatively low cost. Several non-intrusive diagnostic techniques have been proposed to measure the quality of combustion in internal combustion engines. Among these techniques, those based on the analysis of accelerometer data have earned a greater success.
The U.S. Pat. No. 6,388,444 discloses a method for detecting misfires, comprising the steps of measuring engine vibration energy caused by combustion by analyzing accelerometer data, measuring instantaneous crankshaft and camshaft positions for determining in which of the combustion chambers of a multiple cylinder engine combustion is expected to occur, and determining whether or not a normal combustion has occurred using both accelerometer data and crankshaft acceleration data.
Essential features of this prior method are the deployment of an accelerometer for sensing vibrations of the engine, a variable reluctance sensor for sensing acceleration of the crankshaft, and the combined processing of the signals generated by the above devices for detecting misfires.
The U.S. Pat. No. 6,273,064 discloses a method wherein the engine vibration data sensed by an accelerometer is sampled during a defined observation window in the combustion cycle during which combustion occurs. The window is calculated using camshaft and crankshaft position sensor data. The accelerometer data are processed to estimate cylinder combustion energy. This computed value is compared to normal combustion energy values with stable combustion. If the computed value deviates more than a desired amount, spark timing, air/fuel ratio or exhaust gas recirculation are adjusted.
The data processing according to these methods is relatively burdensome and expensive. A need remains for a simple and low-cost technique for sensing a misfire or a partial combustion condition in an engine or, basically, for identifying the angular location of the in-cylinder pressure peak (LPP).